The influence of age and dental status on elevator and depressor muscle activity

The objective of this study was to determine whether the muscle activity at various mandibular positions is affected by age and dental status. Thirty edentulous subjects (E), 20 young dentate individuals (G1) and 20 older dentate individuals (G2) participated in this study. Surface electromyographic (EMG) recordings were obtained from the anterior temporal (T), masseter (M) and depressor muscles (D). Muscle activity was recorded during maximal voluntary contraction (MVC), maximal opening (O(max)) and in six different mandibular positions. One way anova and the Bonferroni tests were used to determine the differences between groups. Significant differences between the three tested groups were found at MVC and O(max) for all examined muscles (P < 0.001). The differences in muscle activity in dentate subjects of different age were found in protrusion for depressor muscles (P < 0.05) and in lateral excursive positions for the working side temporal (P < 0.05) and non-working side masseter and depressor muscle (P < 0.05). There was a significant effect regarding the presence of natural teeth or complete dentures in protrusion and maximal protrusion for all muscles (P < 0.05) and in lateral excursive positions for non-working side temporal (P < 0.05) and working side masseter muscle (P < 0.05). Muscle activity at various mandibular positions depends greatly on the presence of the prosthetic appliance, as edentulous subjects had to use higher muscle activity levels (percentages of maximal EMG value) than age matched dentate subjects in order to perform same mandibular movement. Different elevator muscles were preferentially activated in the edentulous subjects when compared with dentate group in lateral excursive positions of the mandible. The pattern of relative muscle activity was not changed because of ageing.

Effect of treatment of dentofacial deformities on the electromyographic activity of masticatory muscles

Individuals with dentofacial deformities present changes in masticatory muscles. The objective of the present study was to determine the influence of interdisciplinary treatment in cases of class III dentofacial deformities regarding the EMG activity of the temporal (T) and masseter (M) muscles. The study was conducted on 15 patients with class III dentofacial deformities who were submitted to orthodontic, oromyofunctional and surgical treatment and assigned to groups P1 (before surgery) and P2 (6-9 months after surgery). Fifteen individuals with no alterations in facial morphology or dental occlusion and without signs or symptoms of temporomandibular joint dysfunction were used as controls (CG). The T and M muscles were submitted to EMG bilaterally in the situations of mastication and mastication plus biting, with analysis of amplitude interval and root mean square. For all muscles tested, there was a difference between CG, P1 and P2; CG was higher than P2 and P2 higher than P1 in all situations assessed. We conclude that there was an increase in EMG activity in the T and M muscles after surgical correction of the dentofacial deformity accompanied by interdisciplinary treatment, although the values were still lower than those obtained for CG.

[Development of a functional mandibular model]

This study was conducted to establish a functional mandibular biomechanical model for use in the follow-up biomechanical study of the integrated and fractured mandible. The integrated and dry human mandible was used, and the corresponding maxilla and cranial base was duplicated by resin and plaster. 2-mm silicon rubber was used for simulation of the temporomandibular joint (TMJ) disc. A simulated TMJ and physiological mandibular model was developed by four pairs of muscular loadings (Masseter= 180 N, Temporalis = 190 N, Medial Pterygoideus = 120 N, Lateral Pterygoideus = 40 N) in each muscular center, and the functional loading corresponding with physiological condition was reflected and simulated more realistically when compared with that of the previously reported mandibular models which were developed by occlusion loading or by only one pair of muscles loading. In summary, we have established a functional mandibular model which can be used to analyze the biomechanical behavior in various functional conditions.

Oral behaviors checklist: reliability of performance in targeted waking-state behaviors

AIMS

To assess the consistency of intentional behavioral performance as an index of whether individuals understood the meanings of the behavioral terms of the Oral Behaviors Checklist, which is a self-report scale for identifying and quantifying the frequency of jaw overuse behaviors.

METHODS

Surface electromyography was used to measure bilaterally the activity of the masseter, temporalis, and suprahyoid muscles (for assessment of oral behaviors) and the biceps muscles (reference task of biceps curl) in 27 temporomandibular disorder (TMD) cases and 27 controls. Subjects were asked to perform (1) biceps curls to lift 5 weights, with explanation, and (2) 10 oral behaviors (e.g., "clench", "yawn") without explanation.

RESULTS

Biceps-curl performance resulted in assignments of excellent or very good for linearity-reliability based on inspection and correlation. Test-retest reliability of the 10 performed oral behaviors generally ranged from 0.6 to 0.98 for all 3 muscle groups, and many tasks had reliability coefficients comparable to those for the biceps curl. Across tasks, elevator muscle reliability of cases was 0.87, compared to 0.75 for controls; group values for opening muscles were similar.

CONCLUSION

Individual subjects performed each task at a high level of consistency. Performance was not appreciably altered by being a TMD case versus a control and was not significantly different from the performance level of a reference task, indicating that each individual understood well the meaning of each oral behavior-related word.

Regional difference of blood flow in anesthetized rats during reduced gravity induced by parabolic flight

To examine a hypothesis that change in regional blood flow due to decreased hydrostatic pressure gradient and redistribution of blood during reduced gravity (rG) is different between organs, changes in cerebrocortical blood flow (CBF) and blood flow in the temporal muscle (MBF) with exposure to rG were measured in anesthetized rats in head-up tilt and flat positions during parabolic flight. Carotid arterial pressure (CAP), jugular venous pressure (JVP), and abdominal aortic pressure were also measured simultaneously. In the head-up tilt group, CBF increased by 15 +/- 3% within 3 s of entry into rG and rapidly recovered during rG. MBF also increased, but the change was significantly greater than that of CBF. JVP increased by 1.8 +/- 0.5 mmHg, probably due to loss of hydrostatic pressure gradient, since the measuring point of JVP was 2-3 cm above the hydrostatic indifference point. CAP and abdominal aortic pressure increased by 16.7 +/- 2 and 7.7 +/- 2 mmHg, respectively, compared with the 1-G condition. Muscle vascular resistance [(CAP-JVP)/MBF] decreased on entry into rG, but no significant change was observed in cerebrocortical vascular resistance [(CAP-JVP)/CBF]. In the flat group, no significant change was observed in all the variables. The results indicate that arteriolar vasodilatation occurs in the temporal muscle but not in the cerebral cortex. Thus the blood flow control mechanism at the onset of rG is different between intra- and extracranial organs.

Surface EMG of jaw-elevator muscles and chewing pattern in complete denture wearers

The aim of this study was to investigate the adaptation process of masticatory patterns to a new complete denture in edentulous subjects. For this purpose, muscle activity and kinematic parameters of the chewing pattern were simultaneously assessed in seven patients with complete maxillary and mandibular denture. The patients were analysed (i) with the old denture, (ii) with the new denture at the delivery, (iii) after 1 month and (iv) after 3 months from the delivery of the new denture. Surface electromyographic (EMG) signals were recorded from the masseter and temporalis anterior muscles of both sides and jaw movements were tracked measuring the motion of a tiny magnet attached at the lower inter-incisor point. The subjects were asked to chew a bolus on the right and left side. At the delivery of the new denture, peak EMG amplitude of the masseter of the side of the bolus was lower than with the old denture and the masseters of the two sides showed the same intensity of EMG activity, contrary to the case with the old denture. EMG amplitude and asymmetry of the two masseter activities returned as with the old denture in 3 months. The EMG activity in the temporalis anterior was larger with the old denture than in the other conditions. The chewing cycle width and lateral excursion decreased at the delivery of the new denture and recovered after 3 months.

Cochlear temperature correlates with both temporalis muscle and rectal temperatures. Application for testing the otoprotective effect of hypothermia

CONCLUSIONS

During systemic hypothermia, the internal temperature of the rat cochlea correlates best with the temporalis muscle and rectal temperatures. These positive correlations will be used in future studies to assess the efficacy of mild and moderate hypothermia to protect hearing against the progressive loss caused by electrode insertion in a clinically relevant model of cochlear implantation trauma.

OBJECTIVE

To monitor the internal temperature of the cochlea during induced systemic hypothermia using a reference tissue instead of an internal cochlear temperature probe.

MATERIAL AND METHODS

The temperatures of the cochlea, brain, temporalis muscle and rectum were determined during periods of normothermia (37 degrees C), mild (33 degrees C) and moderate (30 degrees C) hypothermia and slow rewarming in anesthetized adult Fisher rats. These values were compared using statistical analysis to establish the best correlations between the temperature of the cochlea and the temperature at the three other temperature measurement sites.

RESULTS

The strongest correlations with the internal temperature of the cochlea during the induction of mild-to-moderate hypothermia were obtained for the temperatures of the ipsilateral temporalis muscle and rectum.

Automatic evaluation of the exteroceptive suppression of the temporalis muscle activity

AIM

To set up a reliable automated method of allowing to identify the ES1 and ES2 periods of exteroceptive suppression of the temporal muscle (EST), thereby reducing their intra- and inter-individual variability.

MATERIALS AND METHODS

Analyses were performed in 79 healthy subjects with strict inclusion and exclusion criteria. Each individual underwent two separate examinations 1 week apart. Activity of the left and right temporal muscles was recorded in response to unilateral stimulation of the left and right labial commissures. Wave forms were successively rectified, averaged, and filtered; the intersections of the resulting curve with values corresponding to 50%, 60%, 70%, 75%, and 80% of the control period (20 ms preanalysis time) were automatically determined.

RESULTS

All subjects reached a 80% level of attenuation for ES2. The values of ES2 durations were normally distributed. The mean ES1 and ES2 durations didn't vary between the two recording session, and there was a good individual reproducibility from one session to the other. Though relatively high, the inter-individual variability was slightly lower in the second than in the first session.

CONCLUSIONS

Computed analysis of EST may contribute to decrease inter- and intra-individual test variability. Letting patient habituate through a first recording session could increase test sensitivity, too. The obtained values for ES2 duration are normally distributed and well-reproducible at both the population and individual level. We suggest taking into account both ES2 duration and level of attenuation.

Reproducibility of surface EMG in the human masseter and anterior temporalis muscle areas

The aim of this study was to test the hypothesis that surface electromyography (sEMG) recordings, made at mandibular rest position from the masseter and temporalis anterior areas, are intra- and inter-session reproducible. A template was designed and built to permit the correct electrode placement from one session to the next session. A sample of 18 subjects was examined. Two groups, homogeneous for age, sex, and craniofacial morphology were selected. The first group included asymptomatic subjects with no signs or symptoms of temporomandibular joint dysfunction (TMD) and the second group included patients suffering from muscle-related TMD. Data were obtained from different sEMG recordings made at mandibular rest position in the same session and in different sessions, repositioning the electrodes using a template designed for that purpose. The electromyograph used in this, study is part of the EMG K6-I Win Diagnostic System. Results showed that reproducibility of sEMG signals from the masseter and anterior temporalis areas at mandibular rest position is possible.

Modulation of jaw reflexes induced by noxious stimulation to the muscle in anesthetized rats

Previous studies have shown that jaw reflexes and activity patterns of the jaw muscles were modulated in the presence of jaw muscle pain. However, there is no study comparing the modulatory effects on the jaw reflexes induced by noxious stimulation to the jaw muscle. To clarify this, effects of the application of mustard oil (MO), an inflammatory irritant, into the temporalis (jaw-closing) muscle on (1) jaw-opening reflex evoked by tooth pulp stimulation (TP-evoked JOR) as a nociceptive reflex, (2) jaw-opening reflex evoked by inferior alveolar nerve stimulation as a non-nociceptive reflex and (3) jaw-closing reflex evoked by trigeminal mesencephalic nucleus stimulation as a proprioceptive reflex were investigated in anesthetized rats. The MO application induced suppression of all reflexes, and the effect on the TP-evoked JOR was more prominent than on the other reflexes. To elucidate the involvement of endogenous opioid system for the suppressive effect, a systemic administration of naloxone following the MO application was conducted. The MO-induced suppressive effect on the TP-evoked JOR was reversed by the naloxone administration. The results suggest that noxious stimulation to the jaw muscle modulate jaw reflexes particularly for the nociceptive jaw-opening reflex, and the modulatory effect includes both facilitatory and inhibitory aspects. The results also suggest that pain modulatory systems such as the endogenous opioid system play a crucial role in the suppression of the nociceptive transmissions related to nociceptive reflexes, and in some pathological states, defense reflexes may not be evoked properly.

Modulation of masticatory muscle activity by tongue position

The purpose of this study was to test whether the tongue position affects the electromyographic (EMG) activities of masticatory muscles. We recorded the EMG activities of the masseter and anterior temporalis muscles in 10 skeletal Class I adults. Tongue position was monitored by two pressure transducers embedded in the midpalatal region and the lingual flange of a custom-made acrylic monoblock. We instructed subjects to assume three different tongue positions: rest, superior, and anterior. Friedman's test and Sheffe's F-test were used to statistically examine differences in muscle activities induced by changes in tongue position. Significant differences were found in masseter muscle activity between the rest and anterior positions and in anterior temporalis muscle activity between the rest and both the anterior and the superior tongue positions. We concluded that masticatory muscle activity is affected by tongue position.

Temporalis function in anthropoids and strepsirrhines: An EMG study

The major purpose of this study is to analyze anterior and posterior temporalis muscle force recruitment and firing patterns in various anthropoid and strepsirrhine primates. There are two specific goals for this project. First, we test the hypothesis that in addition to transversely directed muscle force, the evolution of symphyseal fusion in primates may also be linked to vertically directed balancing-side muscle force during chewing (Hylander et al. [2000] Am. J. Phys. Anthropol. 112:469-492). Second, we test the hypothesis of whether strepsirrhines retain the hypothesized primitive mammalian condition for the firing of the anterior temporalis, whereas anthropoids have the derived condition (Weijs [1994] Biomechanics of Feeding in Vertebrates; Berlin: Springer-Verlag, p. 282-320). Electromyographic (EMG) activities of the left and right anterior and posterior temporalis muscles were recorded and analyzed in baboons, macaques, owl monkeys, thick-tailed galagos, and ring-tailed lemurs. In addition, as we used the working-side superficial masseter as a reference muscle, we also recorded and analyzed EMG activity of the left and right superficial masseter in these primates. The data for the anterior temporalis provided no support for the hypothesis that symphyseal fusion in primates is linked to vertically directed jaw muscle forces during mastication. Thus, symphyseal fusion in primates is most likely mainly linked to the timing and recruitment of transversely directed forces from the balancing-side deep masseter (Hylander et al. [2000] Am. J. Phys. Anthropol. 112:469-492). In addition, our data demonstrate that the firing patterns for the working- and balancing-side anterior temporalis muscles are near identical in both strepsirrhines and anthropoids. Their working- and balancing-side anterior temporalis muscles fire asynchronously and reach peak activity during the power stroke. Similarly, their working- and balancing-side posterior temporalis muscles also fire asynchronously and reach peak activity during the power stroke. Compared to these strepsirrhines, however, the balancing-side posterior temporalis of anthropoids appears to have a relatively delayed firing pattern. Moreover, based on their smaller W/B ratios, anthropoids demonstrate a relative increase in muscle-force recruitment of the balancing-side posterior temporalis. This in turn suggests that anthropoids may emphasize the duration and magnitude of the power stroke during mastication. This hypothesis, however, requires additional testing. Furthermore, during the latter portion of the power stroke, the late activity of the balancing-side posterior temporalis of anthropoids apparently assists the balancing-side deep masseter in driving the working-side molars through the terminal portion of occlusion.

Activity of inferior head of human lateral pterygoid muscle during standardized lateral jaw movements

OBJECTIVE

(a) To describe the changes in electromyographic (EMG) activity from selected jaw muscles during a standardized lateral jaw movement with the teeth together, and (b) to investigate the effects on jaw muscle activity of changes in both the rate of lateral jaw movement and the relative magnitude of jaw-closing force.

DESIGN

In 16 healthy volunteers, recordings were made using a jaw-tracking system, of mid-incisor point (MIPT) movements, as well as EMG activity from the contralateral inferior head of the lateral pterygoid muscle (IHLP), and bilateral anterior and posterior temporalis, masseter and submandibular muscles, during lateral jaw movement tasks at two speeds and two closing force levels with the teeth together.

RESULTS

The IHLP was the only muscle to show a consistent increase in activity in association with the outgoing phase of the task and a decrease during the return phase. Under high closing force at slow speed, the EMG activities of the IHLP and bilateral anterior temporalis and masseter muscles were significantly (p < 0.05) higher than those under a low closing force, while there was no significant change (p > 0.05) in bilateral posterior temporalis and submandibular muscles. The change from slow to fast lateral movement at low force did not significantly (p > 0.05) alter the mean activity except for the IHLP (increase in activity) and the contralateral anterior temporalis (decrease in activity).

CONCLUSIONS

The data suggest that the IHLP is one of the principal jaw muscles involved in a lateral jaw movement with the teeth together while the other jaw muscles may play a contributory or facilitatory role.

Exteroceptive reflexes in jaw-closing muscle EMG during rhythmic jaw closing and clenching in man

Exteroceptive jaw reflexes might play a role in normal functions of the mouth such as mastication. Until now these reflexes have only been studied under isometric conditions. The aim of this study was to compare exteroceptive reflexes in jaw muscle EMG during the closing phase of rhythmic open-close movements and clenching, at the same jaw gape and with similar muscle EMG. Reflexes consisting of successive waves of decreased and increased muscle activity (the Q, R, S and T waves of the post-stimulus electromyographic complex (PSEC)), evoked by light noxious electrical stimulation of the vermillion border of the lower lip, were recorded from the jaw closing muscles of 17 subjects. Differences between the two tasks occurred in two phases of the PSEC: (1) in an early phase, around the R wave, there was significantly less EMG during jaw closing (mean EMG ratio between jaw-closing and clenching 0.71), and (2) in a late phase, around the transition between the S to the T wave, there was significantly more EMG during jaw closing (mean EMG ratio: 1.40). The decrease in EMG activity around the R wave during jaw closing may be due to a change in reflex sensitivity at an interneuron level. The increase in EMG activity around the transition between the S and T waves during jaw closing might, at least in part, be due to a proprioceptive stretch reflex. This reflex is mediated by muscles spindles that are activated by the deceleration of the jaw evoked by the lip stimulus. The finding of inhibitory reflex mechanisms that predominate more during rhythmic jaw movements than during clenching in an early phase of the PSEC might be related to protecting oral tissues from trauma when the jaw is closing with potentially a large muscle force. In contrast, when food is held between the teeth, a possible inhibitory influence of light noxious stimuli is diminished.

Functional influence of masticatory muscles on the fibre characteristics and capillary distribution in growing ferrets (Mustela putonusfuro)—a histochemical analysis

OBJECTIVE

Changes in the masticatory function of ferrets have been shown to cause alterations in their craniofacial morphology. The aim of the present study was to investigate how the functional changes influence the histochemical characteristics of fibres of the masticatory muscles of the animal.

DESIGN

Forty five-week old male ferrets were divided into two groups: one group was fed a diet of hard pellets (HD group), and one group was fed the same diet but softened with water (SD group). After 6 months, specimens of temporalis, masseter and digastricus were taken from each ferret and prepared for enzyme histochemical analysis

RESULTS

Type I muscle fibres constituted the majority of the temporalis and the masseter, while the digastricus mainly comprised type II muscle fibres. The fibre-type composition did not differ significantly between the two groups. The mean cross-sectional area of type I and type II fibres in the temporalis and the masseter, and type II fibres in the digastricus was significantly greater in the HD group than the SD group. Furthermore, the HD group had a significantly greater number of capillaries per fibre, but not per surface unit (mm2).

CONCLUSIONS

The results suggest that the muscle fibre size and the number of capillaries per fibre of the masticatory muscles are readily adaptesd to the altered masticatory muscle functions.

Relationships among nocturnal jaw muscle activities, decreased esophageal pH, and sleep positions

The purpose of this study was to examine the relationships among nocturnal jaw muscle activities, decreased esophageal pH, and sleep positions. Twelve adult volunteers, including 4 bruxism patients, participated in this study. Portable pH monitoring, electromyography of the temporal muscle, and audio-video recordings were conducted during the night in the subjects' homes. Rhythmic masticatory muscle activity (RMMA) episodes were observed most frequently, with single short-burst episodes the second most frequent. The frequencies of RMMA, single short-burst, and clenching episodes were significantly higher during decreased esophageal pH episodes than those during other times. Both the electromyography and the decreased esophageal pH episodes were most frequently observed in the supine position. These results suggest that most jaw muscle activities, ie, RMMA, single short-burst, and clenching episodes, occur in relation to gastroesophageal reflux mainly in the supine position.

Human jaw muscle strength and size in relation to limb muscle strength and size

The aim of the present study was to investigate to what extent general factors (e.g. genotype, hormones) and factors at the craniofacial level (e.g. craniofacial size, jaw muscle architecture) contribute to the size and strength of the jaw muscles. A strong relationship of jaw muscle size and strength with that of other muscles would argue for general influences, whereas a weak relationship would argue for craniofacial influences. In 121 adult individuals, moments of maximal bite force, arm flexion force and leg extension force were measured. In addition, thicknesses of jaw muscles, arm flexor muscles and leg extensor muscles were measured using ultrasound. Relationships were assessed by using a principal component analysis. In females, one component was found in which all force moments were represented. Bite force moment, however, loaded very low. In males, two components were found. One component loaded for arm flexion and leg extension moments, the other loaded for bite force moments. In both females and males, only one component was found for the muscle thicknesses in which all muscle groups loaded similarly. It was concluded that the size of the jaw muscles was significantly related to the size of the limb muscles, suggesting that they were both subject to the same general influences. Maximal voluntary bite force moments were not significantly related to the moments of the arm flexion and leg extension forces, suggesting that besides the general influence on the muscle size, variation in bite force moment was also influenced by local variables, such as craniofacial morphology.

Influence of age on adaptability of human mastication

The objective of this work was to study the influence of age on the ability of subjects to adapt mastication to changes in the hardness of foods. The study was carried out on 67 volunteers aged from 25 to 75 yr (29 males, 38 females) who had complete healthy dentitions. Surface electromyograms of the left and right masseter and temporalis muscles were recorded simultaneously with jaw movements using an electromagnetic transducer. Each volunteer was asked to chew and swallow four visco-elastic model foods of different hardness, each presented three times in random order. The number of masticatory cycles, their frequency, and the sum of all electromyographic (EMG) activity in all four muscles were calculated for each masticatory sequence. Multiple linear regression analyses were used to assess the effects of hardness, age, and gender. Hardness was associated to an increase in the mean number of cycles and mean summed EMG activity per sequence. It also increased mean vertical amplitude. Mean vertical amplitude and mean summed EMG activity per sequence were higher in males. These adaptations were present at all ages. Age was associated with an increase of 0.3 cycles per sequence per year of life and with a progressive increase in mean summed EMG activity per sequence. Cycle and opening duration early in the sequence also fell with age. We concluded that although the number of cycles needed to chew a standard piece of food increases progressively with age, the capacity to adapt to changes in the hardness of food is maintained.

Maximal bite forces in healthy young adults as predicted by surface electromyography

OBJECTIVES

To assess the repeatability of maximal bite force estimates as obtained by submaximal electromyographic-force relationships performed simultaneously and symmetrically in both sides of the mouth. The protocol could be used in a clinical context to obtain indicative values for the occlusal loads to be resisted by the prosthetic reconstructions.

METHODS

Ten young healthy subjects performed; (1) a maximum voluntary clench (MVC) directly on their occlusal surfaces; (2) four simultaneous recording of submaximal bite forces (98, 196, 304 and 392 N on two transducers positioned on the left and right first mandibular molars) and surface EMG potentials of the masseter and temporalis anterior muscles. The actual force peak value was recorded. For each subject, a linear regression was run between the simultaneous bite force and EMG submaximal values recorded in the four tests. Using the EMG potentials obtained during the MVC tests, the best fitting line was used to estimate a maximum bite force. Two independent recordings were made by each subject (2 week interval) and analyzed by correlation analysis, paired Student's t-test, and Dahlberg statistic.

RESULTS

Significant linear relationships were found between bite force and EMG potentials (p<0.01). The two series of indirect estimates of maximal bite force were correlated (r=0.626) without systematic differences (Student's t, p>0.01). Dahlberg statistic was 115.37 N (approximately 36% of the total variance of the group).

CONCLUSIONS

Simultaneously recorded submaximal bite forces and surface EMG potentials of mandibular elevator muscles had a linear relationship. The estimates of maximum bite force were repeatable on a short-term basis (2 weeks). The method limited the disadvantages of bite force recordings, and it could be used to obtain indicative values for the occlusal loads to be resisted by the prosthetic reconstructions.

Response of human jaw muscles to axial stimulation of a molar tooth

The reflexes of the main jaw-closer muscles (masseter and anterior temporalis) on both sides of the jaw were investigated using surface electromyography to observe reflex activity following mechanical stimulation of the 1st right upper-molar tooth at various forces under a number of levels of jaw-muscle activity. As with analogous studies performed on the incisor, three distinct reflex events were identified in the EMG before the earliest conscious subject reaction: early excitation, inhibition and late excitation. However, contrary to observations found during studies on the incisor, excitation, not inhibition was the primary reflex response. The application of a local anaesthetic block around the stimulated molar showed that the primary agents in eliciting the observed reflexes were not contained within the periodontium of the stimulated tooth. A diminished representation of periodontal mechanoreceptors around the molar teeth and more elaborate root structures, hence a more solid connection to the jaw and consequently less tooth movement, were deemed the likely reason for the distinction between the reflex responses of the incisal and molar regions. In addition to the reflex studies, the minimum reaction time of a number of subjects was determined to permit the distinction of a reflex event and an event that could be a conscious subject reaction. It was found that the reaction time of the temporalis muscles was significantly shorter than those of the masseter, while no significant difference was found between the left and right sides. Overall, the data showed that the presence or absence of background muscle activity and subject variability were the main causes of changes in the reflex response, provided the level of the stimulus was greater than 3 N. The application of local anaesthetic had no impact on the reflexes evoked.

Simultaneous modulation of the exteroceptive suppression periods in the trapezius and temporalis muscles by experimental muscle pain

OBJECTIVE

To investigate whether the exteroceptive suppression (ES) periods in the trapezius and temporalis muscles could be simultaneously modulated by either experimental trapezius muscle pain or temporalis muscle pain.

METHODS

Fourteen healthy volunteers participated in two sessions with randomised injections of either hypertonic or isotonic saline into the right trapezius muscle or right temporalis muscle. The pain intensity was continuously scored on a 10 cm electronic visual analogue scale. During muscle contraction, the early (ES1) and late (ES2) reflex in the temporalis and late (ES2) reflex in the trapezius elicited by electrical stimulation of the right infraorbital nerve were recorded pre-, during-, and post-injection.

RESULTS

Hypertonic saline injection induced local muscle pain and referred pain with the maximal pain intensity of 6.3+/-0.5 cm in the right trapezius and 6.7+/-0.6 cm in the right temporalis muscles. Injection of either hypertonic or isotonic saline into the right trapezius muscle was associated with significantly shortened ES2 duration and offset time in the injected right trapezius muscle. Only injection of hypertonic saline into the right trapezius muscle could significantly reduce the ES2 offset time and decrease the degree of suppression of ES2 in the right temporalis and left trapezius muscles. Injection of either hypertonic or isotonic saline into the right trapezius muscle evoked no changes in ES2 parameters in the left temporalis muscle. Injection of hypertonic saline, but not isotonic saline, into the right temporalis muscle caused a significantly shorter ES2 offset time and shorter ES2 duration bilaterally in the temporalis and trapezius muscles.

CONCLUSIONS

The findings of the present study provide evidence for a bi-directional effects reflecting convergence of muscle afferents from the trigeminal and upper cervical neural systems in humans, which may partly underlie the manifestations of pain referral between these two areas.

Prediction of different mandibular activities by EMG signal levels

The purpose of this study was to make observations of electromyographic (EMG) signal levels from a group of individuals to describe different mandibular tasks and use them as predictors of these activities. The sample consisted of 22 volunteer subjects (12 women and 10 men) with mean values for age, weight and height of 29.3 years, 69 kg and 168 cm respectively. Three separate recording sessions were performed for each subject. During each session the subjects were asked to perform different voluntary mandibular tasks. The EMG activity for each task was recorded from the anterior temporalis (T(a)) and masseter muscles (M(m)) bilaterally. The EMG recordings were related to task, session, task-session interaction, gender, age, weight and height using SAS version 6.12. The EMG data from the four muscles were considered as covariates to classify tasks. Overall, 78% of the group tasks were correctly classified. When the masticatory activities were classified as high and low EMG signal level group, the overall classification rate improved with an accuracy of 88%, sensitivity of 86%, specificity of 88%, positive predictive value of 76% and negative predictive value of 94%. The tasks, sessions, task-session interactions, gender, age, weight and height did not have any significant effect on the EMG recordings. It can be concluded that distinguishing among different mandibular tasks of a subject can be achieved when the individual EMG signal levels were compared with the EMG signal levels of subjects that were used to describe different mandibular activities.

The effect of clenching on condylar position: A vector analysis model

STATEMENT OF THE PROBLEM

Clinicians often disagree about the acceptable position of the condyle in the articular fossa. Agreement as to what position the condyle-disk assembly assumes on the articular eminence while subjected to the forces of the elevator muscles of the mandible is important.

PURPOSE

This simulation study investigated the position of the condyle-disk assembly on the articular eminence while subjected to the forces of the elevator muscles of the mandible during clenching.

MATERIAL AND METHODS

Using the 2 conditions of static equilibrium, a vector analysis of a 2-dimensional model of the temporomandibular joint was developed. The dynamics of the condyle-disk assembly during muscle loading were analyzed. The vector sum of the masticatory muscle forces was determined by assuming that the maximum force developed by each contributing muscle was proportional with the physiologic cross section of the muscle. The effect of the resultant force vector of the combined muscle action applied to the condyle was analyzed for the orthogonal and acute angle situations.

RESULTS

When the vector sum was orthogonal on the eminence, the condyle was in stable equilibrium. The stable equilibrium was found in the anterior-superior position in the fossa. When the vector sum was at an acute angle to the eminence, the condyle was subject to a translation force that displaced it into the anterior-superior equilibrium position.

CONCLUSION

Within the limitation of this study, vector analysis suggested that the anterior-superior position of the condyle-disk assembly on the eminence was the most stable equilibrium position. All other tested positions of the condyle were less stable and maintained at the expense of other structures.

Internal architecture, origin-insertion site, and mass of jaw muscles in Old World hamsters

The jaw muscle (i.e., masticatory, suprahyoid, and extrinsic tongue) anatomy and mass were examined in four genera of Old World hamsters (cricetine murids), Mesocricetus, Cricetulus, Tscherskia, and Phodopus. The masseter was the largest and most complicated of the muscles examined. In the superficial layer, a few ventral fibers form a small medially turned portion with an insertion site more similar to those of sciurids than of other murids. In Mesocricetus, the superficial layer has a discrete anteroventral portion that has not been reported for other murid rodents. Examination of the fiber attachment sites indicated that the deep layer contains four parts and the medial layer contains three parts. The deep layer originates from two aponeuroses that are firmly connected to each other at their anterior ends and lie along the zygomatic arch. The aponeurosis of insertion for the deep layer is situated along the masseteric ridge and the dorsal border of the angular process, but is absent in its middle part, consistent with reports in two relatives, sigmodontine and arvicoline murids. In cricetine murids, unlike in other rodents, fibers insert on the dorsal narrow strip of the posterior mandibular aponeurosis, not on its broad medial aspect. The relative mass of some masticatory and suprahyoid muscles is related to body mass. Small species (Cricetulus and Phodopus) have relatively larger masseter and mylohyoid muscles and smaller temporalis and geniohyoid muscles than large species (Mesocricetus and Tscherskia).

Individual variations in numerically modeled human muscle and temporomandibular joint forces during static biting

AIMS

To test the effects of occlusal force (OF) angle on the variations in predicted muscle and temporomandibular joint (TMJ) forces during unilateral molar bites.

METHODS

The craniomandibular (CM) geometries of 21 individuals were determined from lateral and posteroanterior cephalometric radiographs. These geometries were used in a numerical model based on minimization of muscle effort. This model was previously validated for this subject group through the use of jaw tracking and electromyographic data. The model predicted muscle and TMJ forces associated with static OFs on the right mandibular first molar. OF angle was varied from vertical to 40 degrees in the buccal and lingual directions, in increments of 10 degrees.

RESULTS

Intra- and intersubject variations in predicted muscle and TMJ forces for unilateral molar biting were dependent on OF angle and CM geometry. Nonvertical OFs were associated with either large anterior temporalis muscle forces (> 100% of applied OF in 3 subjects) or large inferior lateral pterygoid muscle forces (> 90% of applied OF in 3 subjects). On average, vertically and buccally directed OFs were associated with higher mean contralateral TMJ forces (60% of applied OF, SD 12%). Two subjects had large ipsilateral or contralateral TMJ forces (> 90% of applied OF).

CONCLUSION

In a group of healthy subjects, depending on the individual CM geometry, large muscle and/or TMJ forces were predicted to be associated with specific unilateral molar OF angles. Propensities to increased muscle or joint forces may be predisposing factors in the development of myofascial pain or intracapsular disease. The results may explain, in part, the variation in location of symptoms in individuals who first present with temporomandibular disorders.